In the past, down hole tools have been used to obtain formation fluid samples. These fluids were analyzed by flowing them through a resistivity test chamber. The acidity and temperature of the fluid was also measured.
Down hole sampling tools were suspended by a wireline and lowered into a bore hole. A pair of packers mounted to the tool isolated an interval in the bore hole when expanded into sealing contact with the bore hole wall. Fluid was removed from the isolated interval between the packers, through an opening in the tool, and its resistivity was measured. The resistivity measurement was sent to the surface by a wire line and when the resistivity became constant, indicating that formation fluids uncontaminated by drilling mud components were being withdrawn into the tool, the withdrawn fluids were directed into a separate chamber where the redox potential, acidity and temperature of the fluids were measured. Those results were also sent to the surface by wire line. Depending on the test results, the sample was either retained in a chamber or pumped back into the bore hole. If the sample was rejected, the packers were deflated and the tool shifted to a different position in the bore hole for further sampling This procedure was repeated until all the sample chambers in the tool were filled with the required samples. Such a sampling tool is illustrated in U.S. Pat. No. 4,535,843 entitled Method and Apparatus for Obtaining Selected Samples of Formation Fluids. Since the sampling apparatus in the '843 patent had a purpose solely to obtain formation fluids for analysis and was not used for measuring formation permeability, the sample flow rate into the apparatus was of no concern.
In the past, formation fluid samples were taken through a probe which extended through the bore hole wall and was generally surrounded by a sealing member made from a material compatible with the well fluids. Typically, the fluid opening in the probe was surrounded by an elastomeric annular sealing pad mounted to a support plate which was laterally movable by actuators on the tool. On the opposite side of the tool, a tool anchoring member was selectively extendable for use in conjunction with the movable sealing pad to position the tool in a manner such that the sample point was effectively sealed from well fluids.
Sampling tools used in the past contained pressure sensors. However, there were still concerns about being able to detect during the course of a testing operation whether a sample was actually being obtained and, if a sample was entering the tool, how fast the sample was being admitted to the sample chamber.
Some formation testing tools employed a "water cushion arrangement" with regard to the admission of formation fluids into the tool. As shown in U.S. Pat. No. 3,011,554, this arrangement includes a piston member which is movably disposed in an enclosed sample chamber so as to define upper and lower spaces in the chamber. Where the entrance to the sample chamber is above the piston, the upper space is initially at atmospheric pressure and the lower space is filled with a suitable and nearly incompressible fluid such as water. A second chamber or liquid reservoir which is also initially empty and having a volume equal or greater than the lower space is in flow communication with the lower filled water space by a suitable flow restriction such as an orifice. As formation point. enter the empty upper portion of the sample chamber, the piston is progressively moved formation. from its initial elevated position to displace water from the lower portion of the sample chamber through the orifice and into the initially empty liquid reservoir. formation.
It can readily be seen in this device that the flow control is done by sizing the orifice through which the water from the lower space is displaced into the liquid reservoir downstream of the orifice. This arrangement does not provide for direct control of flow rate of formation fluid into the tool. Depending upon formation permeability and orifice size and initial downstream pressure from the orifice, a situation can arise in such a tool where the pressure drop in the sample line is large enough to cause gas formation when the pressure drops below the bubble point of the formation fluid. When such gas formation occurs, the tool will not yield interpretable results which can be used to determine formation permeability and non representative fluid samples are withdrawn.
Other fluid admission systems have been employed where no water cushion is used. In U.S. Pat. No. 3,653,436, formation fluids were admitted into an initially empty sample chamber. The tool contained a pressure sensor to sense the flow line pressure. The flow line pressure rises imperceptibly at an extremely low rate and it is not until a sample chamber is almost filled that any substantial increase in the measured pressure occurs. In this type of configuration, the fluid sampling rate is not controlled.
A modification of the water cushion type of sampling system is found in U.S. Pat. No. 3,859,850. In the '850 patent, selectively operable valves are opened to place the fluid admitting means into communication with a sample collecting means comprised of an initially empty first collection chamber that is randomly coupled to a vacant accessible portion of a second sample collection chamber that is itself divided by a piston member movably disposed therein and normally biased toward the entrance to the second chamber by a charge of compressed gas confined in an enclosed portion of the second chamber. As sample fluids enter the sample collecting means, the first sample chamber is initially filled before sufficient pressure is built up in the first chamber to begin moving the piston member so as to allow formation fluids to begin filling the second chamber. By observing the time required for filling the first chamber, the flow rate of the entering formation fluids can be estimated.
Once the first chamber is filled and the pressure of formation fluid equals the pressure of the compressed gas, movement of the piston into the gas filled portion of the second chamber further compresses the gas charge so as to impose a proportionally increasing back pressure on the formation fluids which can be measured to obtain a second measurement that may be used to estimate the rate at which formation fluids if any are entering the second sample chamber.
Yet other sampling devices that isolate the sample point from the well fluids at a fixed point on the formation by including a probe surrounded by a resilient seal for sampling formation fluids are described in U.S. Pat. Nos. 3,934,468, and U.K. Patent Application Nos. 2172630A and 2172631A.
In view of the significant expenses involved in drilling oil and gas wells, it is desirable to determine the fluid pressure and permeability of formations in order that the ability of the well to produce can be estimated before committing further resources to the well and at the surface. Most permeable formations are hydraulically anisotropic therefore making it desirable to measure vertical and horizontal permeability for a given formation. This is typically done by creating a pressure gradient in a zone within a selected formation and determining the fluid pressure at one or more points in the zone. The static pressure of a formation is determined at a given point in the formation by the use of a probe having a fluid communication channel between a point in the formation and a suitable pressure measuring device in the bore hole traversing the formation. The formation pressure in the vicinity of the point is changed before, during or after the static pressure measurement to create the gradient zone about that point by passing fluid into or extracting fluid from the formation In U.S. Pat. No. 2,747,401 a dual probe arrangement was illustrated where fluid was either withdrawn or pumped into the formation at one point and pressure gradient measured at another point The measured pressure gradient was representative of the actual and relative permeability of the formation The apparatus in the '401 patent could be used to measure variables permitting calculation of the permeabilities of the formation in several different directions thus revealing the degree of hydraulic anisotropy of the formation
One type of commercially available tool known as RFT by the registered Trade Mark "RFT" (Repeatable Formation Tester) has been used to measure permeability although the tool finds greater application as a pressure measurement device and a sample taker. The problem with this type of tool is that for low permeabilities, the pressure drop caused by the flow at the producing probe was large and gas formation resulted when the pressure dropped below the bubble point of the formation fluid. In such instances, the test was uninterpretable. Conversely, in high permeability situations, the pressure drop was frequently too small and the pressure build up too fast to be measured effectively with commercially available pressure sensors. There have been some modifications of the basic permeability measurement tools. In one such modification, the producing probe pressure drawdown is preset at the surface at a constant value for the duration of the flow. This value can be selected so as to reduce gas formation problems and to maximize pressure amplitude. The problem is that there are no provisions for flow rate measurements nor is the sample size accurately known. Either one of these measurements is necessary to arrive at a reasonable interpretation for the horizontal permeability when the formation is isotropic or only mildly anisotropic i.e., "a" is between 1 and 100 where a=the ratio of the horizontal to the vertical permeability.
In single probe RFT tools, the permeability determined is the spherical or cylindrical permeability. In homogeneous and low anistropy formations, this is sufficient. In heterogeneous or highly anisotropic formations, additional observation probes are necessary for proper formation characterization.
The single probe devices are limited in their usefulness in determining permeability because the depth of investigation is extremely shallow (several inches) during fluid removal. Thus, the information that is gathered from this type of tool only relates to conditions very near the sample point. Such conditions may also be severely altered by the drilling and subsequent fluid invasion process.
Use of multiple probes extended the depth of investigation to a magnitude on the order of the probe spacing.
In order to obtain meaningful permeability information deeper into the formation so as to avoid the effects of drilling damage and formation invasion, the probe spacing must be significantly greater than known designs such as shown in U.S. Pat. No. 2,747,401. Known designs make probe spacing in the order of six to twelve or more feet unworkable since the fluid removal rate and therefore the magnitude of the propagated pressure pulse is limited due to the small bore hole wall area exposed with such tools.
Another way to measure permeability is to use a vertical pulse test. In a cased and cemented well, the casing packer isolates a perforated interval of casing to provide sufficient bore hole area open to flow. This allows a pressure pulse large enough to be measured with a pressure gauge. This type of measurement can only be used after the wall is cased and cemented. Channels behind the casing may alter the effective vertical spacing and therefore the measured results.
The apparatus of the present invention is designed to allow gathering of permeability data over greater depths into the formation than has been possible with prior tools. The apparatus employs a straddle packer as a component of the tool. By allowing greater surface area from which a sample of formation fluid can be taken, larger flow rates can be used and meaningful permeability data for a radius of approximately fifty to eighty feet can be obtained. Additionally, by having the ability to withdraw formation fluid at pressures above the bubble point due to the extended surface area between the packer seals, the spacing between the sample point and the pressure probe is effectively increased to a range of eight to fifteen feet and above thus permitting data collection on formation permeability for points more remote from the tool than was possible with prior designs; providing increased depth of investigation. Additionally, with use of the straddle packer, high accuracy vertical pulse tests can be done using a packer and a single probe.
Additionally, the apparatus of the present invention also employs a flow control feature to regulate the formation fluid flow rate into the tool thereby providing a constant pressure or constant flowrate drawdown on the formation face to enhance the multiprobe permeability determination. With sample flow control, it can be insured that samples are taken above the formation fluid bubble point. Samples can also be taken in unconsolidated zones. The sample flow rate can also be increased to determine the flow rates at which sand will be carried from the formation with the formation fluids.
The apparatus of the present invention can also be constructed to be flexible for doing various types of tests by constructing it in a modular method. Additionally, each module may also be constructed to have a flow line running therethrough as well as electrical and hydraulic fluid control lines which can be placed in alignment when one module is connected to the next. Thus, a tool can be put together to perform a variety of functions while still maintaining a slender profile. Such modules can contain sample chambers, fluid analysis equipment, pressure measurement equipment, a hydraulic pressure system to operate various control systems within the other modules, a packer module for isolating a portion of the well bore from the formation sample point, probe modules for measuring pressure variations during formation fluid sampling and a pump out module to return to the well bore samples that are contaminated with mud cake.